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United States Patent |
5,524,544
|
O'Meara
,   et al.
|
June 11, 1996
|
Nitrocellulose propellant containing a cellulosic burn rate modifier
Abstract
There is disclosed a propellent having a deterred burn rate. The propellent
is a particulate containing a nitrocellulose base and a cellulosic
thermoplastic deterrent, preferably cellulose acetate butyrate or
cellulose acetate propionate. The deterrent is gradationally dispersed
within the particulate with the greatest concentration of deterrent at the
particulate periphery.
Inventors:
|
O'Meara; William L. (Tallahassee, FL);
Murray; Terry A. (Quincy, FL)
|
Assignee:
|
Olin Corporation (St. Petersburg, FL)
|
Appl. No.:
|
278360 |
Filed:
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July 21, 1994 |
Current U.S. Class: |
102/287; 102/289; 102/290; 102/291 |
Intern'l Class: |
C06D 005/06 |
Field of Search: |
149/100
102/287,291,292,290,289
|
References Cited
U.S. Patent Documents
3235425 | Feb., 1966 | Clemens et al. | 148/48.
|
3798085 | Mar., 1974 | Mellow | 149/10.
|
4097316 | Jun., 1978 | Mullay | 149/2.
|
4354884 | Oct., 1982 | Williams | 102/290.
|
4525313 | Jun., 1985 | Muller | 149/96.
|
4597994 | Jul., 1986 | Bolinder et al. | 102/290.
|
4654093 | Mar., 1987 | Bolinder et al. | 102/290.
|
4841863 | Jun., 1989 | Bourgeois | 102/283.
|
4842659 | Jun., 1989 | Mezger et al. | 149/88.
|
4886560 | Dec., 1989 | Cartwright | 149/12.
|
4950342 | Aug., 1990 | Canterberry | 102/290.
|
5269224 | Dec., 1993 | Gonzales et al. | 102/288.
|
5398612 | Mar., 1995 | Graham et al. | 102/287.
|
Other References
Modern Plastics, Mid-Oct. 1991. Resins and Compounds, Cellulosics.
Publication by Eastman Kodak Company (1990) Cellulose Esters.
|
Primary Examiner: Nelson; Peter A.
Attorney, Agent or Firm: Rosenblatt; Gregory S.
Claims
We claim:
1. A propellant, comprising:
a plurality of particulates that burn from a periphery inward, each said
particulate containing a nitrocellulose base; and
a cellulosic thermoplastic burn deterrent gradationally dispersed in an
exterior portion of said particulate, said burn deterrent concentration
greatest about the periphery of each said particulate and decreasing
inwardly.
2. The propellent of claim 1 wherein said cellulosic thermoplastic
deterrent is a cellulose acetate.
3. The propellent of claim 2 wherein said cellulosic thermoplastic
deterrent is selected from the group consisting of cellulose acetate
butyrate and cellulose acetate propionate.
4. The propellent of claim 2 wherein said cellulosic thermoplastic
deterrent has a weight average molecular weight of from about 10,000 to
about 100,000.
5. The propellent of claim 4 wherein said cellulosic thermoplastic
deterrent has a weight average molecular weight of from about 12,000 to
about 75,000.
6. The propellent of claim 4 wherein the concentration of cellulose acetate
at a radial point of from about 20% to about 40% inward of the periphery
is essentially zero.
7. The propellent of claim 6 wherein the concentration of cellulose acetate
at a radial point of from about 20% to about 30% inward of the periphery
is essentially zero.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a burn rate modifier for nitrocellulose base
propellants. More particularly, a cellulose acetate deterrent is
gradationally diffused into the propellant, with the maximum concentration
of deterrent on the surface.
2. Description of Related Art
Smokeless propellant powder compositions containing a nitrocellulose base,
either alone or in combination with other propellant babes, represent the
most common gun powder in use today.
When the base is nitrocellulose, the propellant is referred to as a single
base propellant. When the base is a mixture of nitrocellulose and
nitroglycerin, the base is referred to as a double base propellant. When
the base is a mixture of nitrocellulose, nitroglycerin and nitroguanidine,
the propellant is referred to as a triple base propellant.
The propellant base is provided in either spherical (spheroidal) or
modified spherical (oblate spheroids) globular powders manufactured by
either a batch process or a continuous process. There are many different
commercially available types of globular propellant powders, the
difference primarily being in terms of the web (thickness), grain size,
amount of nitroglycerin (an energy booster) and deterrent composition (to
slow burning). The ammunition maker may also look to other powder types
such as extruded or flake if globular powders do not give the proper
ballistics.
The propellant powders have a high burn rate and may require the presence
of a burn deterrent to reduce the initial burn rate of the powder
composition and to impart a burn rate gradient to produce a high
projectile velocity while preventing unduly high chamber pressures.
Among the deterrents known for nitrocellulose base propellants are linear
polyesters as disclosed in U.S. Pat. No. 3,798,085 to Mellow and a
polycaprolactone polymer as disclosed in U.S. Pat. No. 4,950,342 to
Canterberry. Both the Mellow and the Canterberry patents are incorporated
by reference in their entirety herein. Deterrents are classified as either
"plasticizer type" or "barrier type". The plasticizer type deterrent
diffuses into the propellent grains while the barrier type is normally not
capable of diffusion into the propellent grains and coats the surface.
U.S. Pat. No. 4,354,884 to Williams discloses that single and double base
propellants are usually coated from an aqueous solution containing
dissolved deterrent. When the water is driven off, a coating of deterrent
remains behind. With triple base propellants, nitroguanidine is water
soluble and a nonaqueous solvent is required. One suitable solvent is
methyl alcohol. The triple base propellant is immersed in the nonaqueous
solution containing dissolved deterrent for a desired time, water rinsed
and dried.
It is desirable to diffuse the deterrent into the propellant grains to
establish a concentration gradient. The concentration of the deterrent is
highest along the outside surface of the propellant grain and decreases to
approximately zero at some point within the propellant grain. This
concentration gradient slows down the burn rate when the propellant grains
are large, reducing ballistic pressure. The burn rate increases as the
size of the propellant decreases, maintaining a constant ballistic
pressure.
If the concentration gradient changes, typically heat causes the deterrent
to migrate inward, a negative ballistic effect occurs. The burn rate at
the surface of the propellent grains increases, leading to increased
ballistic pressure. As the grain size decreases, the increased amount of
deterrent reduces the burn rate, leading to a drop in pressure.
Ballistic stability is the capability of the deterrent to remain in the
original concentration gradient without migrating. A problem with
currently used deterrents is ballistic instability. When exposed to
elevated temperatures (65.degree. C. and higher) the prior art deterrents
migrate, causing the ballistic performance to change.
There exists, therefore, a need for a deterrent for a nitrocellulose base
propellant that has greater ballistic stability than those of the prior
art.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a deterrent for
nitrocellulose base propellants having improved ballistic stability. It is
a second object of the invention to provide a method for depositing the
deterrent into the propellant with a concentration gradient such that the
maximum amount of deterrent is at the surface.
It is a feature of the invention that a cellulosic polymer is utilized as
the deterrent. It is a feature of the invention that the cellulosic
deterrent is placed into the propellent under elevated temperature and
high solvent levels. Yet another feature of the invention is that while
the cellulosic thermoplastic is a barrier type deterrent, by using the
proper solvent and heat, diffusion into the propellant is achieved.
It is an advantage of the invention that the cellulosic deterrent migrates
less than linear polyester and other conventional deterrents. There is
minimal deterrent migration after storage at either room temperature or
elevated temperatures for extended periods of time. Another advantage of
the invention is that the cellulosic thermoplastic deterrents are
compatible with both single base and double base smokeless propellants.
In accordance with the invention, there is provided a propellant. The
propellant contains a particulate having a nitrocellulose base. A
cellulosic thermoplastic deterrent is gradationally dispersed in an
exterior portion of the propellant particulate. The deterrent
concentration is greatest about the periphery of the particulate and then
decreases inwardly.
In accordance with a second embodiment of the invention, there is provided
a method for the manufacture of a propellant. An aqueous suspension
containing nitrocellulose particulate is heated to a temperature of from
about 30.degree. C. to about 70.degree. C. A nonaqueous solution
containing from about 1% to about 25% by weight dissolved cellulosic
thermoplastic is then added to the aqueous solution. The
nonaqueous/aqueous solution mix is then agitated for a time effective for
the nonaqueous solution to penetrate at least partially into the
particulate. Substantially all the solvent component of the nonaqueous
solution and the water are then removed to produce cellulosic
thermoplastic coated particulate.
The above-stated objects, features and advantages will become more apparent
from the specification and drawings which follow.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows in cross-sectional representation a propellant particulate
containing the deterrent of the invention.
FIG. 2 graphically illustrates the concentration gradient of the deterrent
after storage for 30 days at 20.degree. C.
FIG. 3 graphically illustrates the concentration gradient of the deterrent
after storage for 30 days at 70.degree. C.
DETAILED DESCRIPTION
FIG. 1 shows in cross-sectional representation a particulate 10 according
to the present invention. The particulate 10 contains a nitrocellulose
base, constituting a single base smokeless powder and may optionally
contain nitroglycerine as a double base smokeless powder. When
nitroglycerin is present, the weight ratio of nitroglycerin to
nitrocellulose is from about 1:1 to about 1:9.
While FIG. 1 illustrates the particulate 10 as a sphere, the particulate
may be any desired shape. Spheroidal (sphere like), oblate spheroidal
(football shaped), cylindrical, equiaxed polyhedron, flake or ribbon are
suitable examples. Notwithstanding the shape of the particulate 10, the
particulate 10 has a periphery 12 defining the outermost surface.
Spheres and other fluent shapes are most preferred for ease of loading a
cartridge. When the particulate 10 is a sphere, the radius 14 is from
about 100 microns to about 4000 microns. Preferably, the radius 14 is from
about 100 to about 1000 microns. When other shapes are utilized, the
particulate size is that effective to achieve an approximately similar
volume of particulate.
Dispersed within the particulate 10 is a deterrent 16. The deterrent 16 is
gradationally dispersed in an exterior portion of the particulate 10 such
that the concentration of deterrent is greatest around the periphery 12
and decreases inwardly towards the center 18 of the particulate.
Preferably, the weight percent concentration of deterrent relative to
propellent is from about 10% to about 50% at the periphery 12. More
preferably, the weight percent of deterrent is from about 10% to about 30%
at the periphery 12. The weight percent decreases inwardly toward the
center 18 and approaches zero at a point between 20% and 40% along the
radius 14, being closer to the periphery 12 than the center 18. More
preferably, the point of essentially zero deterrent is from about 20% to
about 30% of the way inwardly along the radius 18.
The preferred deterrents are cellulosic thermoplastics constituting
repetitive utilization of the anhydroglucose unit (C.sub.6 H.sub.10
O.sub.5). The preferred cellulosic thermoplastics are cellulose esters
formed by mixing cellulose with the appropriate organic acids, acid
anhydrides and catalysts. Preferred materials include cellulose acetates
such as cellulose acetate butyrate and cellulose acetate propionate.
Generic chemical formulas for the preferred deterrents are:
##STR1##
The preferred cellulose esters are not fully acylate, but contain from
about 3% to about 12% by weight of hydroxyl. These materials are available
from Eastman Chemical Products of Kingsport, Tenn.
The cellulosic thermoplastics have a weight average molecular weight in the
range of from about 10,000 to about 100,000 mass units and preferably from
about 12,000 to about 75,000 mass units and are compatible with both
nitrocellulose and nitroglycerin.
The particulate 10 is manufactured by first preparing an aqueous suspension
containing nitrocellulose. The water to propellent ratio (by weight) is
from about 2:1 to about 20:1 and preferably from about 5:1 to about 15:1.
The solution is added to a mixing vessel, and heated and agitated until
the contents form a slurry at a temperature of from about 30.degree. C. to
about 70.degree. C. and preferably from about 40.degree. C. to about
60.degree. C. If nitroglycerin is required for a double base powder, the
desired amount of nitroglycerin is gradually added at this time. The
contents are then mixed under approximately constant agitation and
temperature for a period of from about 60 minutes to about 240 minutes.
When the aqueous suspension is well mixed, the cellulosic deterrent is
added. The cellulose deterrent is dissolved in ethyl acetate or another
suitable nonaqueous solvent. The amount of the cellulosic thermoplastic in
the nonaqueous solvent is between about 1% and about 25% by weight and
preferably between about 5% and 20%. The nonaqueous solution is then added
to the mixing vessel over an extended period of time, typically from about
5 minutes to about 120 minutes.
The temperature of the mixing vessel is then increased to from about
40.degree. C. to the boiling temperature of the nonaqueous solvent,
72.degree. C. for ethyl acetate. The deterrent penetrates into the
propellent grains with the aid of the ethyl acetate. Unlike linear
polyester and other plasticizing deterrents, the cellulosic deterrent does
not diffuse by its own plasticizing action. As a result, the melting
temperature of the cellulosic deterrent is not critical as with linear
polyesters. A preferred temperature range for the vessel during the ethyl
acetate penetration step is between about 50.degree. C. and 70.degree. C.
The temperature and agitation are maintained for the amount of time
required for the desired amount of penetration, typically from about 1
minute to about 480 minutes, and preferably from about 30 minutes to about
120 minutes.
Preferably, the weight percent of deterrent in dried propellant is between
about 0.5% and 10% by weight and preferably from about 3% to about 7% by
weight of the dried propellent.
At the end of the deterrent contact period, the agitating vessel and
contents are heated to a temperature effective to separate the ethyl
acetate from the aqueous solution. The effective temperature is preferably
between about 72.degree. C. and 90.degree. C. Distillation is continued
for a period of time necessary to remove substantially all the ethyl
acetate, typically between about 1 hour and 12 hours.
Following removal of the ethyl acetate, the coated propellent is rolled to
a desired web and the water is removed. The water is removed by heating or
other suitable means such as vacuum assisted heating. Heating to a
temperature of from about 60.degree. C. to about 80.degree. C. for from
about 2 to about 12 hours is satisfactory. Preferably, the water content
is less than about 1% by weight and preferably less than about 0.5%-0.75%
by weight.
To enhance flow, the outside of the dried deterred propellant is preferably
coated with a small amount, typically less than 1% and preferably from
about 0.1-0.5% by weight graphite.
The advantage of the propellant of the invention will be more apparent from
the examples which follow. The examples are illustrative and not intended
to limit the scope of the invention.
EXAMPLES
A nitrocellulose base substantially spheroidal propellent having a radius
of 0.338 mm (0.0133 inch) was coated with either a linear polyester
deterrent or a cellulose acetate butyrate deterrent. The maximum
concentration of deterrent was at the periphery of the spheroidal
propellant. The propellent concentration gradient decreased, approximately
linearly, to about zero percent at a point along the radius about 20%
inward from the outer periphery 12.
The propellants were stored at either 20.degree. C. or 65.degree. C. for 30
days. The concentration gradient of the deterrent was then analytically
mapped using Fourier Transform Infrared Microscope Spectrometry.
FIG. 2 graphically illustrates the concentration gradient of the propellent
samples stored for 30 days at 20.degree. C. There was almost no change in
the concentration gradient of the propellent containing the cellulosic
deterrent of the invention as indicated by reference line 20 or of the
linear polyester deterrent as indicated by reference line 22 when compared
to the deterrent gradient of as-formed propellent.
FIG. 3 graphically illustrates the concentration gradient for similar
propellants containing similar deterrents stored at 70.degree. C. for 30
days. Reference line 24 shows almost no shift in the concentration
gradient of the cellulosic deterrent while reference line 26 shows a
significant shift in the linear polyester gradient.
It is apparent that there has been provided in accordance with this
invention a nitrocellulose base propellent containing a deterrent having
improved ballistic stability and a method for the manufacture of the
deterred propellent that fully satisfy the objects, features and
advantages set forth hereinbefore. While the invention has been described
in combination with specific embodiments thereof, it is evident that many
alternatives, modifications and variations will be apparent to those
skilled in the art in light of the foregoing description. Accordingly, it
is intended to embrace all such alternatives, modifications and variations
as fall within the spirit and broad scope of the appended claims.
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